1. Field of the Invention
This invention relates generally to echo cancellation circuitry and, more particularly, to impedance compensation circuitry utilizing active, directional couplers.
2. Description of the Prior Art
With the impending availability of digital subscriber telephone loops, the potential exists for virtually echo-free transmission over the loops. The main source of echo-generating reflections will then be found on the analog portion of a completed connection, that is, on customer premises, and will occur primarily because of numerous terminations being simultaneously operative. For example, business customers oftentimes utilize a conference call arrangement wherein two or more transmission paths are bridged or paralleled with minimal compensation for the impedance mismatch caused by bridging. Such mismatches are wideband as well as frequency sensitive and may cause particularly deleterious effects on the overall quality of an all digital plant. Also, residential customers often communicate in a mode wherein two telephones are simultaneously off-hook at one location and the only compensation is provided by termination dependent DC current flowing to the telephone instruments.
Echo or reflection characteristics of a network or a loop transmission facility may be expressed in terms of a quantity called return loss, which is a measure of the impedance mismatch at any point in the network or facility and is proportional to the energy reflected at the irregularity whenever an incident wave impinges on the irregularity. Return loss, in decibels, is expressed by 20 log .vertline.(Z+Z.sub.T)/(Z-Z.sub.T).vertline., where Z is the impedance at the point under consideration and Z.sub.T is a reference impedance. A high return loss indicates Z is closely matched to Z.sub.T and reflections due to this mismatch are minimal; return losses of 20 dB or more are typical of well-matched loops or terminations.
As an example of these return loss principles applied to the specific case of bridged telephone extensions, it is supposed that the reference impedance Z.sub.T equals the characteristic impedance Z.sub.o of the transmission facility. If each telephone path has an off-hook impedance Z.sub.o then, to first order, the bridged impedance is Z.sub.o /2 and a substantial mismatch occurs; the return loss is only about 10 dB.
An echo canceler compensates for the impedance mismatch, typically by sensing the source signal and reinjecting a portion of the source signal into the path traversed by the echo or mismatch signal. When the reinjected signal is the negative of the signal returned from the source of reflection, the effect of the reflection is canceled and the signal source operates into a fully matched network.
Prior art echo cancelers are exemplified by those types of cancelers utilized on long-haul analog facilities. In such systems, bidirectional signals carried over two-wire subscriber loops are split into separate transmit and receive unidirectional signals for long distance transmission. Hybrid circuits are standard and well-known arrangements for achieving this separation, and perfect separation is possible when the hybrid balancing network equals the impedance of the two-wire line. However, since subscriber loops present a wide range of impedances, mismatches, and consequently echoes, do occur.
A reference representative of the prior art two-to-four wire echo cancelers is U.S. Pat. No. 3,500,000 issued Mar. 10, 1970 to Kelly and Logan. The patent discloses an adaptive echo canceler, implemented with transversal filters, operating on analog facilities. In this echo canceler, a portion of the analog signal incoming to the hybrid junction on the four-wire side is passed through a transversal filter with adjustable tap gain controls to synthesize a cancellation signal for subtraction from the signal outgoing on the other four-wire path. The resultant outgoing signal is clipped and correlated with the sequence of samples of the incoming signal appearing at the taps of the transversal filter to form control signals for the tap gains of the filter. Because of the variability of subscriber loop impedances, the echo circuitry can become extremely complex and the number of taps required is prohibitively large. Moreover, the circuitry as disclosed only operates in a two-to-four wire environment.